1. Field of the Invention
This invention relates to an illumination system for use in conjunction with wire bonding equipment used in the fabrication of semiconductor devices.
2. Brief Description of the Prior Art
In the fabrication of semiconductor devices, one of the operations generally required involves the bonding of wires between two bonding locations, generally between bond pads on the semiconductor device and lead fingers of a lead frame. This bonding operation requires the use of a wire bonder. A wire bonder uses fine gold wire to couple the bond pads on the semiconductor chip to the individual leads or pins to which connection is to be made, such as, but not limited to, the lead fingers of the lead frame. With the continued diminution in the geometry of semiconductor devices as well as the components within and on the semiconductor devices, it has become increasingly difficult to accurately locate the bond pads and leads or pins to which connection must be made. Bond pads are now approaching dimensions of 5 mils or 25 microns. It is necessary that the bonder know precisely where the connections are to be made on a chip by chip basis since the bond locations can differ from chip to chip, even on essentially identical chips.
To mitigate this problem, a machine-vision system was developed to locate the exact positions where these connections or bonds are to be made. The vision system automatically magnifies, displays, measures and locates different parts of the device at which bonds are to be made. This vision system includes mainly electronics, software, camera, optics and illumination units. Since ambient light is generally insufficient to properly illuminate the chip and lead frame components to which the bond is to be made, illumination units are required to provide the required light to make visible the device to which a bond is to be made while the camera takes pictures through the optics. This picture is digitally stored in the electronics hardware and is digitally processed by the software.
To view different parts of the object, different kinds of lighting are required. In general, an axial light is used to view the die surface because the die surface is generally very smooth and reflects incident light without scattering. Accordingly, incident light directed parallel to an optical axis normal to the surface of the die is reflected back almost entirely parallel to the optical axis. An off axis light is used to view the lead frame which has a generally rough surface and scatters light impinging thereon. The light on the lead frame must be applied evenly to avoid dark spots in the picture, the dark spots (e.g. shadows) introducing features which are not actually a part of the structure being viewed and therefore being undesirable. Accordingly, for viewing of the lead frame, incident light comes from around the object and is diffused so that it scatters in all directions and at all angles. The amount of scattering depends upon the surface roughness of the light conductor which is preferably a highly light transmissive plastic, preferably an acrylic such as Plexiglass or a polycarbonate such as Lexan. The degree of diffusion can vary, depending upon the type of product being bonded. Axial light is also referred to herein as "top light" while off axis light is also referred to herein as "ring light".
In general, in prior art illumination systems for use in conjunction with the bonding operation in semiconductor fabrication, two large moving air-cooled light boxes located at an isolated region on the bonding machine are used to generate the required light. This light is collected using light collecting reflectors and is transferred to the working area using flexible fiber optic cables. At the working area, these fiber optic cables are formed into a standard shape to provide the required type of lighting. This method of generating a large amount of light at an isolated area and transferring it to the required location is highly inefficient and expensive.